The present invention relates to a production process for a connecting rod for internal combustion engines, and specifically relates to a strengthening method for inner surfaces of a bearing into which a piston pin and a crankpin fit in sliding and relative rotating connection.
Generally, a connecting rod such as the above comprises a small end formed at an end of a rod-like portion as a main element and a big end formed at the other end of the rod-like portion. A piston pin of a piston is fitted into the small end and a crankpin of a crankshaft is fitted into the big end in sliding and relative rotating connection. The connecting rod converts the reciprocation of the piston to the rotation of the crankshaft. Such a connecting rod repeatedly receives inertial force from the piston and explosive pressure in the combustion chamber, so that fluctuation of the load, such as tension and compression, repeatedly affects each end of the rod (hereinafter collectively referred to as the xe2x80x9cbearing portionxe2x80x9d). Generally, a bearing metal is fixed in the inner surface of the bearing in order to reduce sliding resistance and noise. In this case, there is a concern that relative sliding between the inner surface of the bearing portion and the bearing metal will occur by the fluctuating load which impinges on the bearing portion, and that fretting corrosion will occur. The fretting corrosion causes remarkable decrease of fatigue strength. Therefore, the thickness of the bearing portion must be additionally designed in consideration of the decrease in the fatigue strength, so that a design to reduce the weight of the apparatus is difficult. A structure in which the bearing metal is omitted for the sake of a light-weight design is known. In this structure, the piston pin and the crankpin are directly supported by the inner surface of the bearing portion, and a lubricating oil is supplied through oil paths, which are formed in the connecting rod, to the sliding surface. In this structure, the sliding portion tends to be galling and seize, and there was a problem that it was difficult to ensure sufficient fatigue strength.
Carburizing and quenching processing has been employed to ensure or improve fatigue strength. The carburizing and quenching processing is a heat processing method in which a product formed of steel with a relatively low carbon content is heated in a carburizing gas comprising CO gas and CH4 gas, etc, as a main component so as to disperse and permeate carbon into the surface layer of the product, and then the product is quenched so as to harden only the surface layer. However, oxygen-containing components such as H2O, CO2, and CO are also included in the carburizing gas, so that oxygen permeates into the surface layer during carburization. The oxygen combines with elements such as Cr, Mo, Si, etc., which are originally added for improving quenching properties, and oxides precipitate at the grain boundaries in the steel. As a result, a grain boundary oxidized layer, which is relatively soft, is formed to a depth of about 50 to 10 xcexcm, and the quenching properties of the surface layer after the carburization is deteriorated, and desired fatigue strength cannot be obtained.
Japanese Patent Application Laid Open (Kokai) No. 62-203766 discloses a process for improving fatigue strength by the carburizing and quenching processing. According to the process, the grain boundary oxidized layer (referred to as the xe2x80x9cabnormal carburized layerxe2x80x9d in the publication) is removed by chemical dissolution after the carburizing and quenching processing, and then shot-peening is performed on the treated surface. Japanese Patent Application Laid Open No. 6-71520 discloses a process in which the grain boundary oxidized layer is removed by CBN (cubic boron nitride) grinding, and then shot-peening is performed on the ground surface. In both processes, the grain boundary oxidized layer must be removed, and the portion in which the compressive residual stress has been formed by the quenching is exposed on the surface, and then the shot-peening is performed thereon, so that new compressive residual stress is imparted.
The quenching depth according to the carburizing and quenching processing, that is, the depth to which the compressive residual stress is formed, varies according to the condition of the carburizing and quenching processing (temperature, carbon concentration, processing time, etc.). It will be the best for the surface hardening processing if the hardest portion, in which the compressive residual stress exhibits the maximum value, can be exposed as a surface. However, in the conventional processes, the processing of fresh surface, from which the grain boundary oxidized layer is removed or the surface on which the shot-peening was performed, is not always the portion which exhibits the highest compressive residual stress. According to the research of the inventors, the depth to which the compressive residual stress exhibits the maximum value is approximately 100 to 200 xcexcm, and is deeper than the usual grain boundary oxidized layer (approximately a depth of 10 to 50 xcexcm, as mentioned above). Therefore, it has not been possible for the surface to exhibit the maximum value of the compressive residual stress even if the grain boundary oxidized layer is removed and shot-peening is performed thereon.
It is therefore an object of the invention to provide a production process for a connecting rod for internal combustion engines, in which the fatigue strength is remarkably improved by an optimum surface hardening processing and the problem of the sliding properties, such as galling and seize, can also be solved.
The present invention provides a production process for a connecting rod for internal combustion engines, the process comprising a carburizing and quenching processing for forming a carburized layer in a surface layer of a material of a connecting rod for internal combustion engines, a grinding processing for grinding the carburized layer formed in a bearing portion of the material until the depth at which compressive residual stress exhibits the maximum value, and a shot-peening processing of the surface ground by the grinding processing.
In the invention, first the carburizing and quenching processing is performed on the material of the connecting rod which is formed of steel with low carbon content. By the carburizing and quenching processing, a carburized layer, which is provided with compressive residual stress and is hardened, is formed at the surface layer of the material. Carburizing and quenching processing in the invention refers to a series of processes in which the material is retained in the carburizing gas at 900xc2x0 C. or more, for example, and then the material is rapidly cooled and tempered. A grain boundary oxidized layer exists in the outermost surface of the formed carburized layer. FIG. 1 shows the characteristics of the surface layer after the carburizing and quenching processing is performed on low-carbon steel, the hardness, the proportion of retained austenite, and the compressive residual stress. These characteristics are approximately as shown in FIG. 1, even though some differences will be observed according to differences in processing conditions. In FIG. 1, the portion indicated by the oblique lines on the left shows the grain boundary oxidized layer. According to FIG. 1, the hardness is constant over some interval from the surface, and it becomes lower below a certain depth. The proportion of the retained austenite is high in the vicinity of the surface, and decreases as the depth increases. The compressive residual stress is small in the vicinity of the surface, increases as the depth increases, and then decreases after it exhibits the maximum value at a certain depth. After the carburizing and quenching processing in which such characteristics are exhibited, the grinding processing for grinding the carburized layer formed in the bearing portion of the material until the depth at which compressive residual stress is at the maximum value is performed. That is, the portion in which the compressive residual stress is the maximum is exposed on the surface of the bearing portion. In the subsequent shot-peening processing, shot-peening is performed on the ground surface of the bearing portion. By the shot-peening processing, new compressive residual stress is provided and a large number of fine concavities is formed on the surface of the bearing.
According to the invention, by grinding the carburized layer of the bearing portion and exposing the portion in which the compressive residual stress exhibits the maximum value as a surface, and by performing shot-peening on the surface, the largest compressive residual stress which can be regarded as being obtainable can be imparted to the bearing portion. Therefore, remarkable improvement in fatigue strength can be obtained. Furthermore, the large number of concavities formed by the shot-peening processing can efficiently employed as an oil reservoir. As a result, sliding properties and lubrication can be improved, and the occurrence of problems such as fretting corrosion, galling, and seize can also be solved, and the galling and problems of seize can be prevented. It should be noted that the bearing portion I the invention refers to at least one of a small end and a large end of a connecting rod, and to an inner surface of a bearing bore which fits with the shaft elements such as a piston pin or a crankpin in sliding and relative rotating connections.
Conventional means for the shot-peening processing, in which shot (particles) strike the surface using hydrostatic pressure, such as that of air or water, and the surface is mechanically impacted (peening), can be applied in the invention. Materials for the shot may be chosen from glass and steel, and materials are specifically chosen to obtain the optimum peening effect. It is desirable that a large number of arcuate concavities be formed on the shot-peened surface for the oil reservoir for the lubricating oil. Therefore, the shape of the shot is desirably spherical and size of the shot is chosen from a range in which a large number of fine concavities can be formed. In the invention, the shot-peening processing is performed to the ground surface of the bearing portion, but may also be performed over the entire surface of the connecting rod. Portions other than the bearing portion are not ground. That portion is treated with carburizing and quenching, and a grain boundary oxidized layer exists thereon. Therefore, the surface is provided with compressive residual stress and hardness. When the shot-peening processing is performed over the entire surface, the bearing portion is simultaneously shot-peened. In this case, the process for avoiding impacts on the portion other than the bearing portion is omitted, and there is an advantage in that productivity is improved.
The effect of the shot-peening processing, in which a large number of fine concavities are used for an oil reservoir with improvement in fatigue strength according to the obtaining of the compressive residual stress, has already been mentioned. According to the research by the inventors, it was discovered that the depth of the carburized layer remaining after grinding processing affects the compressive residual stress and the surface roughness after the shot-peening processing. It should be noted that the xe2x80x9cdepthxe2x80x9d refers to the depth, from the surface which appears after the grinding processing and exhibits the maximum compressive residual stress, to the deepest portion of the carburized layer. The inventors have found that the compressive residual stress after shot-peening processing is ensured at high levels, and concavities having uniform shapes are formed thereby, making the roughness uniform to a fine degree, when the depth of the remaining carburized layer is 0.15 mm or more. Therefore, according to a preferred aspect of the invention, the grinding processing of the carburized layer after the carburizing and quenching processing may be performed so that the carburized layer remains at 0.15 mm or more. The surface roughness obtained by the shot-peening processing is made uniform so that lubricating oil is uniformly supplied over the entire bearing portion, and the sliding properties are improved.
Furthermore, the inventors also discovered through research that the shot-peening pressure in the shot-peening processing affected the surface roughness, which was uniform when the shot-peening pressure was 60 MPa or more, preferably in the range of 100 to 150 MPa. Therefore, according to a preferred aspect of the invention, the shot-peening pressure in the shot-peening processing after the grinding processing is 60 MPa or more. By performing the shot-peening processing at the shot-peening pressure on the ground surface of the bearing portion, the surface roughness is made uniform. Therefore, the lubricating oil is uniformly supplied over the entire bearing portion, and the sliding properties are remarkably improved. In the case in which the shot-peening processing is performed, not only on the bearing portion, but also on the entire connecting rod, as mentioned above, the grain boundary oxidized layer can be certainly removed when the above shot-peening pressure is applied. It should be noted that the xe2x80x9cshot-peening pressurexe2x80x9d refers to the pressure in the fluid used to strike the shot on the object plane; it is air pressure when the fluid is air, and it is water pressure when the fluid is water.